| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

The Quest for Randomness; Chat Wars; Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved); More Info; Words Found in Voynich Manuscript; An Inventory Of 3D Space-Filling Curves; Foxes Using Magnetic Fields To Hunt; 5 Rounds RC5 No Rotation; JP Morgan and Madoff; Ori - Secure, Distributed File System; Physical Unclonable Functions (PUFs); Prejudice on Reddit; Recursion OK; Optimizing Julia Code; Cash Handouts in Brazil; Couple Nice Music Videos; It Also Works!; Adaptive Plaintext; It Works!; RC5 Without Rotation (2); 8 Years...; Attack Against Encrypted Linux Disks; Pushing Back On NSA At IETF; Summary of Experimental Ethics; Very Good Talk On Security, Snowden; Locusts are Grasshoppers!; Vagrant (OpenSuse and IDEs); Interesting Take On Mandela's Context; Haskell Cabal O(n^2) / O(n) Fix; How I Am 4; Chilean Charity Supporting Women; Doing SSH right; Festival of Urban Intervention; Neat Idea - Wormholes Provide Entanglement; And a Link....; Simple Encryption for Python 2.7; OpenSuse 13.1 Is Better!; Little Gain...; More Details on Technofull Damage; Palmrest Cracked Too....; Tecnofull (Lenovo Support) Is Fucking Useless; The Neuroscientist Who Discovered He Was a Psychopath; Interpolating Polynomials; Bottlehead Crack as Pre-amp; Ooops K702!; Bottlehead Crack, AKG K701; Breaking RC5 Without Rotation; Great post thank you; Big Balls of Mud; Phabricator - Tools for working together; Amazing Julia RC5 Code Parameterized By Word Size; Chi-Square Can Be Two-Sided; Why Do Brits Accept Surveillance?; Statistics Done Wrong; Mesas Trape from Bravo; European Report on Crypto Primitives and Protocols; Interesting Omissions; Oryx And Crake (Margaret Atwood); Music and Theory; My Arduino Programs; Elliptic Curve Crypto; Re: Licensing Interpreted Code; Licensing Interpreted Code; ASUS 1015E-DS03 OpenSuse 12.3 SSD; translating lettuce feature files into stub steps files; Re: translating lettuce feature files into stub steps files

© 2006-2013 Andrew Cooke (site) / post authors (content).

I Just Wrote a Regular Exression Engine!

From: "andrew cooke" <andrew@...>

Date: Sat, 14 Mar 2009 22:02:50 -0300 (CLST)

Heh.  I just finished a regular expression engine in Python.  It's the
"real deal" in that it "compiles" to a finite state machine (so it runs in
time proportional to the length of the string to be matcher).  It doesn't
compress multiple character jumps into a single step, but it does
otherwise generate a compact machine (as far as I understand these
things).

Being pure Python it's both better and worse than the standard "re"
package.  In fact it's mainly worse - it must be slower, it doesn't match
sub-expressions, and it has a very, very simple syntax.

But it does have a few advantages.  First, it's a generator, so it yields
each match as it finds it.  Second, it takes a sequence, rather than a
string as an argument, which means that the entire string doesn't have to
be read into memory.  Third, I understand it and can take it apart and
extend it, which means I can add Python functions to it.  I could even
make it work with arbitrary lists (non-characters) pretty easily.

Actually, as I implemented this, I realised that there were various things
about the standard Python regexp implementation that I didn't understand
that well, so some of the above may be wrong.  Next thing to do is to look
more closely at the standard library (yes, perhaps I should have started
that way, but way back then I didn't know what to ask).

Here's the test I just got running.  Note that the matcher (the FSM) takes
a list of regexps, and that each has a tag (here, integers).  The results
include the tags.  Also, that's the full regexp syntax - all I support is
literal characters, ranges, and "*".

  def test_all(self):
    regexps = [_parser(1, 'a*'),
               _parser(2, 'a[a-cx]*'),
               _parser(3, 'aax')]
    fsm = Fsm(regexps)
    results = list(fsm.all_for_string('aaxbxcxdx'))
    assert results == [(1, ''), (1, 'a'), (2, 'a'),
                       (1, 'aa'), (2, 'aa'), (3, 'aax'),
                       (2, 'aax'), (2, 'aaxb'),
                       (2, 'aaxbx'), (2, 'aaxbxc'),
                       (2, 'aaxbxcx')]

The source will be in the next LEPL release.

Andrew

Corrected Test

From: "andrew cooke" <andrew@...>

Date: Sat, 14 Mar 2009 22:11:57 -0300 (CLST)

The test I meant to write (includes () grouping for *):

  def test_all(self):
    regexps = [_parser(1, 'a*'),
               _parser(2, 'a([a-c]x)*'),
               _parser(3, 'aax')]
    fsm = Fsm(regexps)
    results = list(fsm.all_for_string('aaxbxcxdx'))
    assert results == [(1, ''), (1, 'a'), (2, 'a'),
                       (1, 'aa'), (2, 'aax'), (3, 'aax'),
                       (2, 'aaxbx'), (2, 'aaxbxcx')]

Also, apologies for typos in text/title.  Given the nature of this
(email-based) blog it's too much effort to always be correcting things...

Andrew

Incomplete

From: "andrew cooke" <andrew@...>

Date: Sun, 15 Mar 2009 10:34:59 -0400 (CLT)

Ooops.  I had ignored embedded alternatives, only allowing a choice at the
start, thinking that I was not losing anything.  But in fact that means
the current implementation has no backtracking.  Fortunately, I don't
think it will be hard to extend.

Andrew

Possibly Complete

From: "andrew cooke" <andrew@...>

Date: Sun, 15 Mar 2009 21:08:14 -0400 (CLT)

Hmm.  I implemented choices without thinking that much, and now it strikes
me that there is no backtracking - the FSM just transitions away...  I
guess that makes sense?  I need to sleep on it.

Anyway, here's the current test:

    regexps = [unicode_parser(1, 'a*'),
               unicode_parser(2, 'a([a-c]x|axb)*'),
               unicode_parser(3, 'aax')]
    fsm = SimpleFsm(regexps, UNICODE)
    results = list(fsm.all_for_string('aaxbxcxdx'))
    assert results == [(1, ''), (1, 'a'), (2, 'a'), (1, 'aa'),
                       (2, 'aax'), (3, 'aax'), (2, 'aaxb'),
                       (2, 'aaxbx'), (2, 'aaxbxcx')], results

(The initial list could now be written as a single expression, except that
there is no way to specify a label in-line).

Andrew

Does Make Sense

From: "andrew cooke" <andrew@...>

Date: Sun, 15 Mar 2009 21:34:24 -0400 (CLT)

Of course it makes sense - my FSM is deterministic (which means it may
need exponential size for the lookup table in certain cases).

Also, I don't have "epsilon"?  Am I still incomplete?  I think so....
Perhaps best to add it with "?"?  In fact, perhaps I do have epsilon if I
just relax the parser to accept, for example, "(a|)"...

Should probably add "." and "^" too (although both those clearly sugar).

Andrew

Comment on this post